Pacing to the left atrium is important for successful bi-atrial pacing. The pacing to the left atrium is usually accomplished by placing a pacing lead into the coronary sinus, which is a venous structure accessible through the right atrium of the heart and serves to drain the coronary veins. The coronary sinus is generally wider at its ostium and tapers inwardly away from the ostium towards the distal portions of the coronary sinus. The ostium of the coronary sinus is located at the juncture of the right atrium and the right ventricle.
To pace the left atrium, a pacing lead can be positioned so that an electrode contacts the wall of the coronary sinus closest to the left atrium. Because the coronary sinus is in electrical contact with the left atrium, by pacing the coronary sinus at this position, one can also pace the left atrium. The pacing lead generally is advanced to the ostium of the coronary sinus through the right atrium portion of the right heart. For effective pacing, it is desirable that a tip electrode on the pacing lead is placed directly into constant contact with the left atrial side of the coronary sinus. By having this constant contact, high voltages, and thus instability and high thresholds, can be inhibited.
To accomplish such constant contact, different pacing lead configurations have been used to assist in the placement and retention of the pacing lead in the desired position. For example, leads have been used in which a body of the lead is pre-formed to have a sinusoidal or helical configuration enabling the lead to expand into contact with the walls of the coronary sinus and retain the lead. Examples of such pre-formed coronary sinus leads are disclosed in U.S. Pat. No. 5,423,865 to Bowald et al. and U.S. Pat. No. 5,476,498 to Ayers.
Referring to FIGS. 1a-1f, specific examples of pre-formed coronary sinus leads are depicted and described in U.S. Pat. No. 6,321,123 to Morris et al, which is incorporated herein by reference. Pacing leads 20 according to Morris et al. generally comprise a first curved portion 22, a second curved portion 24, a tip electrode 26, and additional electrodes 28. A sheath is used to insert the lead 20 into a coronary sinus. Once the sheath and lead 20 are within the coronary sinus, the sheath is removed and the lead 20 takes its pre-formed shape. The pre-formed “J” in the lead 20 can cause the tip 26 to be pressed up against the wall of the coronary sinus, but only if the width of the pre-formed pacing lead 20 is greater than that of the coronary sinus. When the coronary sinus is wider than the pacing lead 20, the tip electrode can lose contact with the wall of the coronary sinus. This can lead to higher voltage requirements during pacing, and thus higher instability and pacing threshold values.
Referring to FIG. 2, there are also other pacing leads 30 that can be used to pace the left atrium through the coronary sinus, such as the Medtronic Attain® Bipolar OTW Lead Model No. 4194 and leads as disclosed in U.S. Pat. No. 5,683,445 to Swoyer, both of which are incorporated herein by reference. The pacing lead 30 generally includes a first curved portion 32, a second curved portion 34, and a tip electrode 36. As depicted in FIG. 2, the angle 31 at the first curve 32 is greater than ninety degrees. A sheath is used to insert the lead 30 as depicted in FIG. 2 into a coronary sinus. Once the sheath and lead 30 are within the coronary sinus, the sheath is removed and the lead 30 takes its pre-formed shape. This enables the tip electrode 36 to be contact the walls of the coronary sinus. However, using these leads to pace the coronary sinus presents the same problems inherent with the leads according to Morris et al. in that when the coronary sinus is wider than the pacing lead, the tip electrode can lose contact with the wall of the coronary sinus leading to instability and higher pacing voltages and threshold values
There is currently a need for a lead assuring stable pacing of the left atrium through the coronary sinus. Because the general problems discussed above have not been addressed by conventional pacing leads, there is a current need for pacing leads addressing the problems and deficiencies inherent with conventional designs.